Chinese president Hu Jintao today broke new ground in his nation's action on climate change, but failed to deliver measures that would significantly stir up the stagnant negotiations towards an international treaty to fight global warming.

For the first time, Hu promised to set a target for reducing the rate at which China's greenhouse gas emissions are rising. The pledge to reduce China's so-called carbon intensity means total emissions will still grow, but the fossil fuel burned for each rise in economic growth will fall by a "notable margin" by 2020, compared with 2005 levels. It represents a concession to rich nations such as the US who were demanding some kind of action before taking on deep cuts themselves.

"Out of a sense of responsibility to the world and its people," Hu said, China has tackled climate change and will continue to do so. UN secretary Ban Ki Moon thanked Hu for his "important commitment" but Hu failed to set a figure for the intensity cut, or a date by which China's total emissions would start falling. Hu reiterated China's goal of 15% of its energy to be renewable by 2020, to plant 40m hectares of trees and to offer aid to the poorest developing nations facing the impacts of climate change.

A few moments before, US President Barack Obama had spoken strongly on the need to act on global warming but offered no new proposals that could jumpstart stalled talks on a UN climate pact. He faces severe opposition at home against proposed laws to cut US emissions.

In his speech, Obama said time was running out to address the problem. "Our generation's response to this challenge will be judged by history, for if we fail to meet it — boldly, swiftly, and together — we risk consigning future generations to an irreversible catastrophe," he said. "The time we have to reverse this tide is running out."

Observers had hoped the United States and China — the world's biggest polluters — would inject momentum, just over two months before 190 nations gather in Copenhagen aiming to complete a climate treaty.

Moon, who called the unprecedented meeting, said talks were moving too slowly. "Failure to reach broad agreement in Copenhagen would be morally inexcusable, economically short-sighted and politically unwise," Ban said.

But Michael Levi, from the Council on Foreign Relations in the US, was disappointed with Hu's speech. "Doesn't seem that he made the much-anticipated significant announcement that people were hoping for," he tweeted.

Rajendra Pachauri, chairman of the UN Intergovernmental Panel on Climate Change that shared the Nobel Peace Prize with former vice president Al Gore in 2007 had reminded the leaders that "the science leaves us with no room for inaction now."

Today's UN summit and the G20 summit in Pittsburgh later this week have been seen as a last chance for world leaders to accelerate the complex negotiations. These resume next week in Bangkok, where officials will aim to turn 200 pages of draft agreement, heavily caveated, into more meaningful document.